Repetition coding for a wireless system

ABSTRACT

A system and method are disclosed for transmitting data over a wireless channel. In some embodiments, transmitting data includes receiving convolutionally encoded data and enhancing the transmission of the data by further repetition encoding the data.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/666,952 entitled REPETITION CODING FOR A WIRELESS SYSTEMfiled Sep. 17, 2003 which is incorporated herein by reference for allpurposes.

FIELD OF THE INVENTION

The present invention relates generally to a data transmission schemefor a wireless communication system. More specifically, a repetitioncoding scheme for a wireless system is disclosed.

BACKGROUND OF THE INVENTION

The IEEE 802.11a, 802.11b, and 802.11g standards, which are herebyincorporated by reference, specify wireless communications systems inbands at 2.4 GHz and 5 GHz. The combination of the 802.11a and 802.11gstandards, written as the 802.11a/g standard, will be referred torepeatedly herein for the purpose of example. It should be noted thatthe techniques described are also applicable to the 802.11b standardwhere appropriate. It would be useful if alternate systems could bedeveloped for communication over an extended range or in noisyenvironments. Such communication is collectively referred to herein asextended range communication. The IEEE 802.11a/g standard specifies arobust data encoding scheme that includes error correction. However, forextended range communication, a more robust data transmission scheme atreduced data rates is required.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1A is a diagram illustrating the data portion of a regular802.11a/g OFDM packet.

FIG. 1B is a diagram illustrating the data portion of a modified802.11a/g OFDM packet where each symbol is repeated twice (r=2).

FIG. 2A is a diagram illustrating a transmitter system with a repetitionencoder placed after the output of an interleaver such as the onespecified in the IEEE 802.11a/g specification.

FIG. 2B is a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system depicted in FIG. 2A.

FIG. 3A is a diagram illustrating a transmitter system with a repetitionencoder placed before the input of an interleaver designed to handlerepetition coded bits such as the one described below

FIG. 3B is a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system depicted in FIG. 3A.

FIGS. 4A-4C are tables illustrating an interleaver.

DETAILED DESCRIPTION

It should be appreciated that the present invention can be implementedin numerous ways, including as a process, an apparatus, a system, or acomputer readable medium such as a computer readable storage medium or acomputer network wherein program instructions are sent over optical orelectronic communication links. It should be noted that the order of thesteps of disclosed processes may be altered within the scope of theinvention.

A detailed description of one or more preferred embodiments of theinvention is provided below along with accompanying figures thatillustrate by way of example the principles of the invention. While theinvention is described in connection with such embodiments, it should beunderstood that the invention is not limited to any embodiment. On thecontrary, the scope of the invention is limited only by the appendedclaims and the invention encompasses numerous alternatives,modifications and equivalents. For the purpose of example, numerousspecific details are set forth in the following description in order toprovide a thorough understanding of the present invention. The presentinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the present invention is notunnecessarily obscured.

In a typical system as described below, bits representing a set of datathat is to be communicated are convolutionally encoded or otherwisetransformed into values. Various types of modulation may be used such asBPSK, QPSK, 16QAM or 32QAM. In the case of BPSK, which is describedfurther herein, each BPSK symbol may have one of two values and eachBPSK symbol corresponds to one bit. An OFDM symbol includes 48 valuesthat are transmitted on different subchannels. To provide extendedrange, each value that is sent is repeated several times by thetransmitter. In one embodiment, the bits are convolutionally encodedusing the same encoding scheme as the encoding scheme specified for theIEEE 802.11a/g standard. Each encoded value is repeated and transmitted.Preferably, the values are repeated in the frequency domain, but thevalues may also be repeated in the time domain. In some embodiments, therepetition coding is implemented before interleaving and a speciallydesigned interleaver is used to handle repeated values. In addition, apseudorandom code may be superimposed on the OFDM symbols to lower thepeak to average ratio of the transmitted signal.

The receiver combines each of the signals that correspond to therepetition coded values and then uses the combined signal to recover thevalues. In embodiments where the values are combined in the frequencydomain, the signals are combined coherently with correction made fordifferent subchannel transfer functions and phase shift errors. For thepurpose of this description and the claims, “coherently” combiningshould not be interpreted to mean that the signals are perfectlycoherently combined, but only that some phase correction is implemented.The signals from different subchannels are weighted according to thequality of each subchannel. A combined subchannel weighting is providedto a Viterbi detector to facilitate the determination of the most likelytransmitted sequence.

Using the modulation and encoding scheme incorporated in the IEEE802.11a/g standard, the required signal to noise ratio decreaseslinearly with data rate assuming the same modulation technique and basecode rate are not changed and repetition coding is used. Some furthergains could be achieved through the use of a better code or outer code.However, in a dual mode system that is capable of implementing both theIEEE 802.11a/g standard and an extended range mode, the complexityintroduced by those techniques may not be worth the limited gains thatcould be achieved. Implementing repetition of values is in comparisonsimpler and more efficient in many cases.

The repetition code can be implemented either in the time domain or inthe frequency domain. For time domain repetition, the OFDM symbols inthe time domain (after the IFFT operation) are repeated a desired numberof times, depending on the data rate. This scheme has an advantage inefficiency since just one guard interval is required for r-repeated OFDMsymbols in the time domain.

FIG. 1A is a diagram illustrating the data portion of a regular802.11a/g OFDM packet. Each OFDM symbol 102 is separated by a guard band104. FIG. 1B is a diagram illustrating the data portion of a modified802.11a/g OFDM packet where each symbol is repeated twice (r=2). Eachset of repeated symbols 112 is separated by a single guard band 104.There is no need for a guard band between the repeated symbols.

The OFDM symbols can also be repeated in the frequency domain (beforethe IFFT). The disadvantage of this scheme is that one guard intervalhas to be inserted between every OFDM symbol in the time-domain sincethe OFDM symbols with frequency-domain repetition are not periodic.However, repetition in the frequency domain can achieve better multipathperformance if the repetition pattern is configured in thefrequency-domain to achieve frequency diversity.

In a typical environment where signals are reflected one or more timesbetween the transmitter and the receiver, it is possible that certainreflections and direct signals will tend to cancel out at the receiverbecause the phase difference between the paths could be close to 180degrees. For different frequencies, the phase difference between thepaths will be different and so spreading the repeated values amongdifferent frequencies to achieve frequency diversity ensures that atleast some of the values will arrive at the receiver with sufficientsignal strength to be combined and read. To maximize the benefit offrequency diversity, it is preferable to repeat values acrosssubchannels that are as widely spaced as is practicable, since the phasedifference between adjacent subchannels is small.

FIG. 2A is a diagram illustrating a transmitter system with a repetitionencoder placed after the output of an interleaver such as the onespecified in the IEEE 802.11a/g specification. In this example system,BPSK modulation is implemented and the repetition encoder and theinterleaver are described as operating on bits, which is equivalent tooperating on the corresponding values. In other embodiments, othermodulation schemes may be used and values may be repeated andinterleaved. The interleaver is included in the IEEE 802.11a/gtransmitter specification for the purpose of changing the order of thebits sent to remove correlation among consecutive bits introduced by theconvolutional encoder. Incoming data is convolutionally encoded byconvolutional encoder 202. The output of convolutional encoder 202 isinterleaved by IEEE 802.11a/g interleaver 204. Repetition encoder 206repeats the bits and pseudorandom mask combiner 208 combines the outputof repetition encoder 206 with a pseudorandom mask for the purpose ofreducing the peak to average ratio of the signal, as is described below.The signal is then processed by IFFT processor 210 before beingtransmitted.

FIG. 2B is a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system depicted in FIG. 2A. Thereceived signal is processed by FFT processor 220. The output of FFTprocessor 220 is input to mask remover 218 which removes thepseudorandom mask. Data combiner 216 combines the repetition encodeddata into a stream of nonrepetitive data. The operation of data combiner216 is described in further detail below. IEEE 802.11a/g deinterleaver214 deinterleaves the data and Viterbi decoder 212 determines the mostlikely sequence of data that was input to the transmission systemoriginally.

The system depicted in FIGS. 2A and 2B can use the same interleaver anddeinterleaver as the regular 802.11a/g system, and also has flexibilityin designing the repetition pattern since the repetition coder is placedright before the IFFT block. However, it has certain disadvantages. Datapadding is required at the transmitter and data buffering is required atthe receiver. Bits have to be padded according to the number of bytes tobe sent and the data rate. The number of padded bits is determined byhow many bits one OFDM symbol can carry. Since the 802.11a/g interleaverworks with 48 coded bits for BPSK modulation, bits need to be padded tomake the number of coded bits a multiple of 48. Since the repetitioncoder is placed after the interleaver, it may be necessary to pad thedata by adding unnecessary bits for lower data rates than 6 Mbps.

For example, one OFDM symbol would carry exactly 1 uncoded repeated bitat a data rate of ¼ Mbps. Since the OFDM symbol could be generated fromthat one bit, there would never be a need to add extra uncoded bits andso padding would not be necessary in principle. However, due to thespecial structure of the 802.11a/g interleaver, several bits would needto be padded to make the number of coded bits a multiple of 48 beforethe interleaver. The padded bits convey no information and add to theoverhead of the transmission, making it more inefficient.

On the other hand, if the repetition encoder is placed after theinterleaver, the repetition coded bits generated from the 48 interleavedbits are distributed over multiple OFDM symbols. Therefore, the receiverwould need to process the multiple OFDM symbols before deinterleavingthe data could be performed. Therefore, additional buffers would benecessary to store frequency-domain data.

The system can be improved and the need for data padding at thetransmitter and data buffering at the receiver can be eliminated byredesigning the interleaver so that it operates on bits output from therepetition encoder.

FIG. 3A is a diagram illustrating a transmitter system with a repetitionencoder placed before the input of an interleaver designed to handlerepetition coded bits such as the one described below. Incoming data isconvolutionally encoded by convolutional encoder 302. The output ofconvolutional encoder 302 is repetition coded by repetition encoder 304.Interleaver 306 interleaves the repetition coded bits. Interleaver 306is designed so that data padding is not required and so that for lowerrepetition levels, the bits are interleaved so as to separate repeatedbits. Pseudorandom mask combiner 308 combines the output of Interleaver306 with a pseudorandom mask for the purpose of reducing the peak toaverage ratio of the signal, as is described below. The signal is thenprocessed by IFFT processor 310 before being transmitted.

FIG. 3B is a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system depicted in FIG. 3A. Thereceived signal is processed by FFT processor 320. The output of FFTprocessor 320 is input to mask remover 318 which removes thepseudorandom mask. Deinterleaver 316 deinterleaves the data. Datacombiner 314 combines the repetition encoded data into a stream ofnonrepetitive data. The operation of data combiner 314 is described infurther detail below. Viterbi decoder 312 determines the most likelysequence of data that was input to the transmission system originally.

Interleaver 306 is preferably designed such that the same (repeated)data are transmitted well separated in the frequency domain to achievefull frequency diversity. For example, a repetition pattern in thefrequency domain for in 1 Mbps mode in one embodiment would repeat eachbit 6 times. Denoting data in the frequency domain as d₁, d₂, . . . ,d₈, the repeated sequence of data is given by:

d₁d₁d₁d₁d₁d₁d₂d₂d₂d₂d₂d₂ . . . d₈d₈d₈d₈d₈d₈

The same data are placed in a group fashion because it is easy tocombine those data at the receiver. Note that the repeated data can becombined only after r (6 in this example) data are available.

The repetition pattern in the above example does not provide thegreatest possible frequency diversity since the spacing between the samedata transmitted on adjacent subchannels may not be large enough and thesubchannels corresponding to the same data are not completelyindependent. Greater frequency diversity would be desirable especiallyfor multipath channels with large delay spreads. Interleaver 306,therefore, is designed to spread the repeated data in the frequencydomain to achieve frequency diversity as much as is practical.

In one embodiment, the interleaver is designed to optimize the frequencydiversity provided by the interleaver for data rates faster than 1 Mbps(repetition number <=6). For lower data rates ½ and ¼ Mbps, there isenough repetition that sufficient subchannels are covered to providefrequency diversity even if adjacent subchannels are used. In thepreferred interleaver described below, repeated bits are separated atleast by 8 subchannels and consecutive coded bits from the convolutionalencoder are separated at least by 3 subchannels. The interleaver isdesigned according to the following steps:

-   -   1. A 6×8 table is generated as shown in FIG. 4A to satisfy the        first rule which specifies that bits are separated at least by 8        subchannels.    -   2. As shown in FIG. 4B, the columns are swapped to meet the        second rule which specifies that consecutive coded bits are        separated at least by 3 subchannels.    -   3. As shown in FIG. 4C, separation between repeated bits is        increased by swapping rows. In the example shown, repeated bits        are separated by at least 16 bins for 3 Mbps (Repetition        number=2 for 3 Mbps so each bit is repeated once.)

For the example interleaver shown, if the input to the interleaver is{1, 2, 3, . . . , 48}, then the output would be: {1, 19, 37, 7, 25, 43,13, 31, 4, 22, 40, 10, 28, 46, 16, 34, 2, 20, 38, 8, 26, 44, 14, 32, 5,23, 41, 11, 29, 47, 17, 35, 3, 21, 39, 9, 27, 45, 15, 33, 6, 24, 42, 12,30, 48, 18, 36}.

Repetition of the values in the frequency domain tends to generate apeak in the time domain, especially for very low data rates (i.e., forlarge repetition numbers). The large peak-to-average ratio (PAR) causesproblems for the system, especially the transmit power amplifier. Thisproblem can be ameliorated by scrambling or masking the valuestransmitted on different frequencies so that they are not all the same.As long as the masking scheme is known, the scrambling can be undone atthe receiver. In one embodiment, the frequency-domain data is multipliedby the long symbol of 802.11a/g, which was carefully designed in termsof PAR. As can be seen in FIG. 2, the mask operation is performed rightbefore the IFFT operation. In general, any masking sequence can be usedthat causes repeated values to differ enough that the PAR is suitablyreduced. For example, a pseudorandom code is used in some embodiments.

At the receiver, decoding includes: (1) mask removal, (2)deinterleaving, (3) data combining, (4) channel correction, (5) Viterbidecoding. It should be noted that in some embodiments, the order of thesteps may be changed as is appropriate.

In embodiments using frequency repetition, the transmitter preferablymasks the frequency-domain signal to reduce the peak-to-average ratio(PAR) in the time-domain. The receiver removes the mask imposed by thetransmitter. If, as in the example above, the mask used by thetransmitter consists of +/−1 s, then the mask is removed by changing thesigns of the FFT outputs in the receiver. After the mask is removed, thedata is deinterleaved according to the interleaving pattern at thetransmitter.

The repeated signal is combined in the frequency domain at the receiverto increase the SNR of the repeated signal over the SNR had the signalnot been repeated. The SNR is increased by multiplying the complexconjugate of the channel response as follows.

$Y_{c} = {\sum\limits_{j \in S_{c}}{H_{j}^{*}Y_{j}}}$$H_{c} = {\sum\limits_{j \in S_{c}}{H_{j}}^{2}}$

where Y_(j) is the signal in subchannel j; H_(j) is the response ofsubchannel j, Y_(c) is the combined signal, H_(c) is the combinedchannel, and S_(c) is the set of indices corresponding to the frequencysubchannels that contain the same data.

The channel effect is preferably removed before the data is input to theViterbi decoder so that the Viterbi decoder is able to use the same softdecision unit regardless of the actual channel response. In theextended-range mode, the combined channel is used in the channelcorrection unit.

The frequency-domain signals are weighted for calculating thepath-metrics in the soft-decision Viterbi decoder, and the optimalweights are determined by the corresponding SNR.

The resulting SNR for the combined signal becomes:

${S\; N\; R} = {\sum\limits_{j \in S_{c}}{{H_{j}}^{2}\frac{E_{x}}{\sigma_{j}^{2}}}}$

where E_(x) is the signal power, and σ_(j) ² is the noise power for thesubchannel j. The combined SNR is used to evaluate the Viterbi weights.

The 802.11a/g standard specifies that there are four pilot signalsincluded in each OFDM symbol for the purpose of estimating timing offsetand frequency offset and tracking phase noise in 802.11a/g signals. The802.11a/g system assumes that these 4 pilots are reliable enough toestimate the phase information. That assumption may not be true for asystem with a very low SNR. The redundancy that exists in thefrequency-domain signal is exploited to help the pilots to estimate andtrack phase.

The phase information is estimated from the frequency domain data asfollows:

1. The repeated signals are combined in the frequency domain to increasethe SNR, with a channel estimate determined from a preamble sequence oflong symbols and an estimated slope, which captures the effect of timingoffset.

2. Hard decisions are made for each of the combined signals afterremoving the phase offset estimated from the previous symbol.

3. The combined signals are multiplied by their own hard decisions. Theaverage of the hard-decision corrected signal is used to evaluate anangle to estimate the phase offset for the current symbol.

A filter is applied to the estimated phase offset to reduce the effectof noise. In one embodiment, a nonlinear median filter is used. Thenonlinear median filter effectively detects and corrects an abruptchange in the phase offset, which could be caused by hard decisionerrors.

An encoding and decoding scheme for a wireless system has beendisclosed. Preferably, repetition coding in the frequency domain isused. An interleaver that provides frequency diversity has beendescribed. In various embodiments, the described techniques may becombined or used separately according to specific system requirements.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and apparatus of the present invention.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

1. (canceled)
 2. A receiver, comprising: an antenna configured toreceive a signal transmitted on a wireless channel; a mask removerconfigured to remove a mask to obtain a de-masked signal; a combinerconfigured to combine data associated with the de-masked signal toobtain a combined signal, including by: obtaining a first set of dataassociated with a first subchannel; obtaining a second set of dataassociated with a second subchannel; determining a first weight based atleast in part on a first measure of quality associated with the firstsubchannel; determining a second weight based at least in part on asecond measure of quality associated with the second subchannel; andcombining the first set of data and the second set of data based atleast in part on the first weight and the second weight; and a decoderconfigured to decode a signal associated with the combined signal. 3.The receiver of claim 2 further comprising a Fast Fourier Transform(FFT) configured to convert a time domain signal into a frequency domainsignal, wherein the frequency domain signal is passed to the maskremover.
 4. The receiver of claim 2 further comprising a de-interleaver,wherein de-interleaving processing is performed before combiningprocessing associated with the combiner.
 5. The receiver of claim 4,wherein the de-interleaver is configured to receive the combined signalfrom the combiner and/or passes the de-interleaved signal to thedecoder.
 6. The receiver of claim 2 further comprising a de-interleaver,wherein de-interleaved processing is performed after combiningprocessing associated with the combiner.
 7. The receiver of claim 6,wherein the de-interleaver is configured to receive the de-masked signalfrom the mask remover and/or passes the de-interleaved signal to thecombiner.
 8. The receiver of claim 2, wherein the decoder includes aViterbi decoder.
 9. The receiver of claim 2, wherein: the signalreceived by the antenna is transmitted by a transmitting device; and thetransmitting device includes a masking module configured to apply a masksuch that the peak to average ratio of the signal output by the maskingmodule is less than the peak to average ration of the signal input bythe masking module.
 10. The receiver of claim 9, wherein the maskapplied includes a pseudorandom value.
 11. A method for processing areceived signal, comprising: receiving a signal transmitted on awireless channel; removing a mask to obtain a de-masked signal;combining data associated with the de-masked signal to obtain a combinedsignal, including by: obtaining a first set of data associated with afirst subchannel; obtaining a second set of data associated with asecond subchannel; determining a first weight based at least in part ona first measure of quality associated with the first subchannel;determining a second weight based at least in part on a second measureof quality associated with the second subchannel; and combining thefirst set of data and the second set of data based at least in part onthe first weight and the second weight; and decoding a signal associatedwith the combined signal.
 12. The method of claim 11 further comprisingperforming Fast Fourier Transform (FFT) processing to convert a timedomain signal into a frequency domain signal, wherein the frequencydomain signal is passed to the mask remover.
 13. The method of claim 11further comprising de-interleaving, wherein the de-interleavingprocessing is performed before combining.
 14. The method of claim 11further comprising de-interleaving, wherein the de-interleavingprocessing is performed after combining.
 15. The method of claim 11,wherein: the received signal is transmitted by a transmitting device;and the transmitting device is configured to perform masking where amask is applied such that the peak to average ratio of the output isless than the peak to average ration of the input.
 16. A computerprogram product for processing a received signal, the computer programproduct being embodied in a computer readable storage medium andcomprising computer instructions for: receiving a signal transmitted ona wireless channel; removing a mask to obtain a de-masked signal;combining data associated with the de-masked signal to obtain a combinedsignal, including by: obtaining a first set of data associated with afirst subchannel; obtaining a second set of data associated with asecond subchannel; determining a first weight based at least in part ona first measure of quality associated with the first subchannel;determining a second weight based at least in part on a second measureof quality associated with the second subchannel; and combining thefirst set of data and the second set of data based at least in part onthe first weight and the second weight; and decoding a signal associatedwith the combined signal.
 17. The computer program product of claim 16further comprising computer instructions for performing Fast FourierTransform (FFT) processing to convert a time domain signal into afrequency domain signal, wherein the frequency domain signal is passedto the mask remover.
 18. The computer program product of claim 16further comprising computer instructions for de-interleaving, whereinthe de-interleaving processing is performed before combining.
 19. Thecomputer program product of claim 16 further comprising computerinstructions for de-interleaving, wherein the de-interleaving processingis performed after combining.